Cartilage consists of a relatively small number of chondrocytes distributed in a matrix composed mainly of type II collagen and a chondroitin sulfate proteoglycan. Previously, we isolated a glycoprotein from chicken serum which also proved to be a component of cartilage. This glycoprotein, chondronectin, mediates the attachment of chondrocytes to type II collagen and also interacts with the glycosaminoglycan moiety of the proteoglycan. We have found taht chondronectin stimulates chondrogenesis in cultured prechondrogenic limb bud mesenchyme and that differentiation in these cultures can be inhibited by antibodies to chondronectin. We have also purified human chondronectin by affinity chromatography and are preparing monoclonal antibodies to the protein in order to investigate the role of chondronectin in human diseases involving cartilage. We are also studying the effects of a protein from testis on the phenotypic stability and differentiation of chondrocytes and certain other cell types. A partially purified fraction inhibits chondrogenesis in chondrocytes and the conversion of preadipocytes into adipocytes. In addition, using fractions of ovine testicular fluid in cultured melanoma cells, we have observed two activities, one which inhibits and one which stimulates differentiation. We are purifying these activities and will use them to study the modulation of differentiation in chondrocytes and other cells. We think dedifferentiation proteins exist in testes fluid which are able to inhibit the development of a broad range of cells.